Metal matrix composites (MMCs), particularly those based upon aluminum alloys, have gained increasing popularity and recognition as alternative structural materials, especially for applications requiring increased stiffness, wear resistance, and strength. MMCs are usually produced by mixing nonmetallic reinforcing particles such as grit, powder, fibers or the like into a metallic matrix. For example, aluminum-based MMCs are composed typically of aluminum alloys (e.g., 6061, 2024, 7075, or A356) reinforced with ceramic particles such as silicon carbide or aluminum oxide (alumina) powder. The reinforcement provided by these particles contributes strength, stiffness, hardness, and wear resistance, in addition to other desirable properties, to the composite.
Despite their growing market, the high cost of manufacturing MMCs has hampered their ability to be priced competitively with unreinforced metallic materials. Traditionally, the fabrication of metal matrix composites has employed non-liquid methods such as the compaction of blends of ceramic particles or fibers and aluminum powders, or the metal spraying of continuous fibers in a lay-up process. Unfortunately, the high cost of metallic powders and the explosion and pyrophoric hazards associated with large quantities of powders have prevented a significant reduction in the cost of MMCs produced by this approach.
Numerous researchers have reported the preparation of MMCs by mixing various ceramic powders and fibers into molten aluminum-based matrices. The equipment and methods utilized in many of these early experiments were extremely simple. The equipment usually consisted of a heated crucible containing molten aluminum alloy and a motor to rotate a paddle-style impeller made of graphite or coated steel in the molten aluminum while ceramic particles were added to the surface of the molten metal (i.e., the melt). The vortex formed by the rotating impeller drew the ceramic particles into the melt and the shear developed between the impeller and the walls of the crucible helped wet the particles. The temperature was usually maintained below the liquidus temperature (in the two-phase region) to keep the aluminum alloy in a semi-solid condition, since the higher viscosity of the partially solid melt further increased the shear force created by the simple impeller. This process has been called compocasting.
Unfortunately, the MMCs made by the compocasting process and other early stir-cast methods suffered from various problems. For example, these early experiments typically involved only small batches. In addition, the processes were performed under atmospheric pressure, using either ambient air or an inert gas to cover the molten metal. In either case, the turbulence created by the mixing process aspirated a significant amount of gas. As a result, the vortex formed by the impeller rotation drew considerable amounts of air or gas down into the melt. Because the composite is sensitive to turbulence and the particles act as sites for the entrapment of gas bubbles, the solidified composites produced by these early processes were often porous. In addition, it was common for the stir-cast or compocast MMCs to contain numerous oxide skins due to the passing of the particles through the surface oxide into the body of the melt. Another problem with the compocasting process was the low level of shear developed by the rotating impeller in the semi-liquid matrix. Since shear is needed for wetting, the particles were generally incompletely wetted by the molten metal alloys. In sum, the quality of the composites produced by these early stir-cast approaches was poor and not considered commercially viable.
The aforementioned compocasting process and other prior stir-cast processing techniques used in the manufacture of metal matrix composite materials are described in detail in U.S. Pat. No. 5,531,425 to Skibo et al., the disclosure of which is incorporated herein by reference. Little or no improvement in these processes occurred until the development of a stir-cast process performed under vacuum, known as the Duralcan process.
Today, Duralcan, a division of Alcan Aluminum Corporation, is a leader in the manufacture and sale of stir-cast aluminum-based MMCs. The technological development which led to the Duralcan process is based on an improvement in mixing efficiency combined with a reduction in gas entrapment. In this process, a low vacuum of approximately 1-5 torr is drawn over molten aluminum heated above the liquidus temperature (in the fully liquid region). The reinforcing particles are added to the surface of the melt and an impeller capable of creating a moderately high level of shear in a low viscosity melt is inserted into the molten metal and stirred at high rotational speed, as measured in revolutions per minute (rpm). The vacuum removes the air which tends to act as a buffer, cushioning the particles and preventing intimate contact with the metal. With the particles in contact with the metal from the start of the process, wetting can begin immediately. The high shear impeller physically shears the particles into the aluminum alloy, spreading the aluminum over the high surface area of the fine particles, thereby rapidly wetting them. The quality of the resulting MMC is much improved over that produced by the other techniques described above. The particles are essentially 100% wetted and there is little or no porosity in the Duralcan MMC. However, while the end product of the Duralcan process is of high quality, the high cost of manufacture, due in large part to the inefficiency of particle mixing and the requirement of costly vacuum equipment, has prevented Duralcan from fully exploiting the potential MMC market.
The Duralcan process is a vacuum batch process that can be divided into three general stages. The first stage is the incorporation of the particles into the molten aluminum, i.e., bringing the particles into intimate contact with the aluminum so that wetting can begin. This stage relies on the formation of a vortex to draw the particles into the body of the melt and a vacuum for eliminating the cushioning effect of gas at atmospheric pressure. In the second stage, the particles must be sheared in the melt through the use of a rotating impeller which produces high shear force. In general, the impeller must have sharp teeth and rotate at sufficient rotational speed in order to break up agglomerates of particles such that each particle may individually come into contact with the aluminum melt. The rotational speed requirement seems to be related to a minimum level of shear generated at a specific surface velocity of the impeller in the melt. Typically, if the rotational speed of the impeller, as measured in rpm, is too low and/or the edges of the teeth are dull, low porosity MMC material comprising well-wetted particles cannot be produced. To further enhance the level of shear, a stationary bar or baffle is positioned proximate to the perimeter of the rotating impeller. A small region of increased shear is created between the outer periphery of the impeller and the baffle. The third stage involves the slow general motion of the composite in the mixing vessel so that substantially all of the composite eventually passes through the area of high shear several times. This motion also ensures uniformity of particle distribution throughout the batch.
However, the Duralcan process, and other similar stir-cast processes practiced presently, have certain shortcomings and disadvantages. In particular, the wetting of the particles, which is the main objective of mixing, begins only when the ceramic particles that are poured on the surface of the molten metal move downward through the matrix towards the rotating impeller. This process proceeds at a slow rate because the vortex is comparatively small and the downward motion is not especially strong; also, localized shear is provided only in the proximity of the baffle. Furthermore, because the ceramic particles are added to the matrix surface, the particle feed rate must be carefully controlled so as to prevent the accumulation of particles on the surface which can, in turn, choke the agitator and further slow the mixing process. Although the impeller and baffle system is simple, rugged, and easy to repair, it is inefficient and does not take advantage of the potential region of high shear which could be made to completely surround the rotating impeller. As a result, the wetting process takes much longer than necessary because the particles must pass through the narrow shear region between the impeller and the baffle several times before the agglomerates are dispersed and the molten aluminum uniformly contacts and wets each particle.
The inefficient mixing of large quantities of MMCs also produces defects in the molten composite. More specifically, agglomerates of incompletely wetted particles may become encased in heavy stable oxide skins which form as the particles roll on the melt surface oxide before submerging and moving towards the impeller. If the oxide coating is thick, the mixing process will sometimes have insufficient intensity to break the agglomerates into individually wetted particles regardless of mixing duration. These partially wetted agglomerates persist after mixing and can lead to internal and surface defects which may be detrimental to properties such as fatigue and fracture. The aluminum oxide skins also have a detrimental effect on the MMC product, because they increase the viscosity of the composite matrix during the casting process and limit the ability to cast intricate shapes having thin walls.
One of the major cost factors in the manufacture of stir-cast aluminum MMCs is the time required to mix particles into molten aluminum so that the individual particles are thoroughly wetted and uniformly distributed in the composite. Prior attempts at increasing the rate of particle wetting and decreasing the process time for particle mixing have not been wholly successful. For example, Sifferlin, in U.S. Pat. No. 3,858,640, describes the introduction of reinforcing particles into molten metal by blowing the particles into the melt using a neutral gas. This process, however, requires large amounts of gas to carry the particles. Thus, the gas becomes entrapped in the composite matrix, which is extremely sensitive to gas and turbulence, and results in a porous composite product. Others have described a process in which the particles are plunged under the surface of the composite melt during mixing with a mechanical hand cylinder. This process, however, produces MMCs with numerous oxide skins since the particles are pushed down through the surface oxide into the body of the matrix.
Another significant cost factor is the use of vacuum which requires that the melting and mixing equipment be encased in a vacuum chamber. Vacuum processing also necessitates additional costly hardware, such as pumps and valves, which complicates the process and increases the time required to make the MMC material.
Thus, there exists a continuing need for an apparatus and method for preparing MMC materials which obviate the sources of increased costs found in the prior manufacturing processes, namely, inefficient mixing of particles and the need for vacuum equipment. The present invention fulfills these needs and further provides related advantages, while avoiding or eliminating many of the problems and shortcomings of the prior art processes. For example, if high quality MMCs could be manufactured under atmospheric or near-atmospheric pressure, such a process could be carried out in many foundries and cast houses where end users could make the MMCs and convert them directly into end products without the need for remelting small ingots with the associated melting costs and melt losses. In addition, an MMC manufacturing process that is performed under atmospheric pressure may be performed as a continuous, rather than batch, process where, for example, ceramic powder and molten aluminum are mixed together and a stream of liquid MMC is produced. A continuous process would dramatically reduce MMC cost as well as provide a way of meeting the potentially enormous MMC market needs.